How quickly does it get dark?

[Tiki]

How would SQM readings be affected in the two hours preceding the onset of astronomical twilight (assume astro twilight 00.39 until 04.34 in lat 51N)?

[acey]

The rate of darkness depends on the angle at which the Sun descends below the horizon: if you're in an equatorial region where the Sun descends at 90 degrees then it gets dark very quickly; if you're at a northern latitude like UK then the Sun in summer sets at a much shallower angle, so we have longer twilight. Check twilight times for any location at a site such as this:

www.timeanddate.com/worldclock/astronomy.html

SQM readings during early twilight are roughly linear in time, with the constant of proportionality being a function of latitude and local conditions. If you're concerned about having sufficient darkness for DSO observing then I would say that full darkness (Sun more than 18 degrees below horizon) is not necessary; 16 degrees is just as good, and 15 degrees is enough for any but the toughest DSOs (this is the limit sometimes called "amateur astronomy twilight"). 13 degrees (one more than the limit of nautical twilight) is about the minimum for effective observing (you should just about see the Milky Way), but some DSO observation (the brightest, easiest objects) is also possible in nautical twilight. This is all assuming it's a dark site of course - a light-polluted one is effectively in permanent twilight.

This paper discusses twilight sky brightness:

http://adsabs.harvard.edu/abs/2006A%26A...455..385P

In practical terms, find out when the Sun will be at least 13 degrees below horizon and observe then. In summer the darkest time is always around 1 a.m. BST. This calculator can be used to find the Sun's altitude (which you want to be negative) for a given location and time:

http://www.pveducation.org/pvcdrom/properties-of-sunlight/sun-position-calculator

[Tiki]

Thanks Acey. I like the idea of 'Amateur astronomy twilight'. The calculator looks good too.

[Tiki]

'Amateur astronomy twilight' begins at 23h 3m 1s in Southampton tonight. The calculator is good.

[Tiki]

23h 33m 1s. Sorry no edit.